Posttraumatic stress disorder (PTSD) is a disabling and costly psychiatric disorder that is estimated to occur in 20% of individuals who are exposed to a traumatic event and is chronic in one third of cases. In addition to its negative impact on quality of life, there is substantial evidence that PTSD (even after controlling for depression and other risk factors) is associated with a markedly increased risk of cardiovascular morbidity and mortality. However, the mechanisms for the association between PTSD and cardiovascular disease (CVD) risk are not well understood. Although adverse health behaviors, including cigarette smoking, alcohol abuse and poor medication adherence are common in PTSD, recent prospective studies show that they do not account for the magnitude of CVD risk among individuals with PTSD. We propose to test our central hypothesis by evaluating whether CBT-I results in improved biomarkers of CVD risk among those with PTSD. Well established biomarkers of CVD related morbidity and mortality will be used including measures of vascular endothelial function measured by brachial artery flow-mediated dilation (FMD), nighttime blood pressure (BP) dipping measured using 24-hour ambulatory blood pressure monitoring (ABPM), and sympathetic nervous system (SNS) activity as measured by 24-hour urinary catecholamines. We will also assess lipid profile, which along with BP is a modifiable component with marked impact on the atherosclerotic cardiovascular disease (ASCVD) risk score. The primary sleep parameter of interest is objectively-measured sleep efficiency (through actigraphy), although self-report insomnia measures and sleep related arousal will also be measured. The rationale for the proposed research is that once it is established that insomnia is an important and modifiable symptom conveying increased CVD risk in this population, the development of new and innovative approaches to integrating insomnia treatment with PTSD-focused interventions can be developed. 150 men and women with comorbid PTSD and insomnia disorder will be randomly assigned with a 2:1 ratio to 8-week cognitive behavioral therapy-Insomnia (CBT-I) intervention or a waiting period control condition. Sleep quality parameters and CVD risk biomarkers will be assessed at pre- randomization baseline, post-intervention, and at a 6-month follow-up. The study is designed to evaluate the association between insomnia and CVD risk biomarkers among persons with PTSD, and determine whether improvements in insomnia symptoms are associated with improvements in CVD risk biomarkers.